Academic Commons Search Resultshttp://academiccommons.columbia.edu/catalog.rss?f%5Bauthor_facet%5D%5B%5D=Naik%2C+Naomi+H.&q=&rows=500&sort=record_creation_date+desc
Academic Commons Search Resultsen-usCoupled climate model simulations of Mediterranean winter cyclones and large-scale ﬂow patternshttp://academiccommons.columbia.edu/catalog/ac:159111
Ziv, B.; Kushnir, Yochanan; Naik, Naomi H.; Harpaz, T.http://hdl.handle.net/10022/AC:P:19738Fri, 12 Apr 2013 00:00:00 +0000The study aims to evaluate the ability of global, coupled climate models to reproduce the synoptic regime of the Mediterranean Basin. The output of simulations of the 9 models included in the IPCC CMIP3 effort is compared to the NCEP-NCAR reanalyzed data for the period 1961–1990. The study examined the spatial distribution of cyclone occurrence, the mean Mediterranean upper- and lower-level troughs, the inter-annual variation and trend in the occurrence of the Mediterranean cyclones, and the main large-scale circulation patterns, represented by rotated EOFs of 500 hPa and sea level pressure. The models reproduce successfully the two maxima in cyclone density in the Mediterranean and their locations, the location of the average upper- and lower-level troughs, the relative inter-annual variation in cyclone occurrences and the structure of the four leading large scale EOFs. The main discrepancy is the models' underestimation of the cyclone density in the Mediterranean, especially in its western part. The models' skill in reproducing the cyclone distribution is found correlated with their spatial resolution, especially in the vertical. The current improvement in model spatial resolution suggests that their ability to reproduce the Mediterranean cyclones would be improved as well.Climate change, Atmospheric sciencesyk16, nhn2Lamont-Doherty Earth ObservatoryArticlesAtmospheric Circulation Response to an Instantaneous Doubling of Carbon Dioxide. Part II: Atmospheric Transient Adjustment and Its Dynamicshttp://academiccommons.columbia.edu/catalog/ac:159148
Shaw, Tiffany Ann; Seager, Richard; Ting, Mingfang; Naik, Naomi H.; Wu, Yutianhttp://hdl.handle.net/10022/AC:P:19748Fri, 12 Apr 2013 00:00:00 +0000The dynamical mechanisms underlying the transient circulation adjustment in the extratropical atmosphere after the instantaneous doubling of carbon dioxide are investigated using the National Center for Atmospheric Research Community Atmosphere Model version 3 coupled to a Slab Ocean Model. It is shown that the transient process during the first few months of integration is important in setting up the extratropical circulation response in equilibrium such as the poleward shift of the tropospheric jet streams. Three phases are found during the transient thermal/dynamical adjustment in the Northern Hemisphere: 1) a radiatively driven easterly anomaly in the subpolar stratosphere, 2) an acceleration of the westerly anomaly in the subpolar stratosphere as a result of anomalous planetary-scale eddy momentum flux convergence, and 3) a “downward migration” of the westerly anomaly from the lower stratosphere to the troposphere, followed by the tropospheric jet shift. Several proposed mechanisms for inducing the poleward shift of the tropospheric jet streams are examined. No significant increase in eddy phase speed is found. The rise in tropopause height appears to lead the tropospheric jet shift but no close relation is observed. The length scale of transient eddies does increase but does not lead the tropospheric jet shift. Finally, the tropospheric jet shift can be captured by changes in the index of refraction and the resulting anomalous eddy propagation in the troposphere.Atmospheric sciences, Climate changetas2163, rs229, mt2204, nhn2Lamont-Doherty Earth Observatory, Earth and Environmental SciencesArticlesA Mechanisms-Based Approach to Detecting Recent Anthropogenic Hydroclimate Changehttp://academiccommons.columbia.edu/catalog/ac:143807
Seager, Richard; Naik, Naomi H.http://hdl.handle.net/10022/AC:P:12332Tue, 24 Jan 2012 00:00:00 +0000Both naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (P 2 E). Natural variability of P 2 E is dominated by the El Niño–Southern Oscillation (ENSO) cycle and is "dynamics dominated" in that the associated global P2E anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced P 2 E change is "thermodynamics mediated" in that the rise in specific humidity leads to intensified patterns of moisture transport and P 2 E. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in P 2 E. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of P 2 E in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed P 2 E trend since 1979, but removal of this influence leaves an emerging pattern of P 2 E change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability.Climate changers229, nhn2Lamont-Doherty Earth ObservatoryArticlesNorthern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10http://academiccommons.columbia.edu/catalog/ac:134161
Seager, Richard; Kushnir, Yochanan; Nakamura, Jennifer A.; Ting, Mingfang; Naik, Naomi H.http://hdl.handle.net/10022/AC:P:10553Mon, 20 Jun 2011 00:00:00 +0000Winter 2009/10 had anomalously large snowfall in the central parts of the United States and in northwestern Europe. Connections between seasonal snow anomalies and the large scale atmospheric circulation are explored. An El NiÃ±o state is associated with positive snowfall anomalies in the southern and central United States and along the eastern seaboard and negative anomalies to the north. A negative NAO causes positive snow anomalies across eastern North America and in northern Europe. It is argued that increased snowfall in the southern U.S. is contributed to by a southward displaced storm track but further north, in the eastern U.S. and northern Europe, positive snow anomalies arise from the cold temperature anomalies of a negative NAO. These relations are used with observed values of NINO3 and the NAO to conclude that the negative NAO and El NiÃ±o event were responsible for the northern hemisphere snow anomalies of winter 2009/10.Environmental sciencers229, yk16, jam148, mt2204, nhn2Lamont-Doherty Earth Observatory, Earth and Environmental SciencesArticlesMechanisms of Tropical Atlantic SST Influence on North American Precipitation Variabilityhttp://academiccommons.columbia.edu/catalog/ac:134152
Kushnir, Yochanan; Seager, Richard; Ting, Mingfang; Naik, Naomi H.; Nakamura, Jennifer A.http://hdl.handle.net/10022/AC:P:10551Mon, 20 Jun 2011 00:00:00 +0000The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of 90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studies with observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO. The observed association between TNA SST anomalies and global and North American precipitation and sea level pressure variability is compared with that found in the output of an atmospheric general circulation model (AGCM) forced with observed SST variability, both globally and in the tropical Atlantic alone. The similarity between model output and observations suggests that TNA SST variability is causal. The mechanisms of the “upstream” influence of the Atlantic on North American precipitation are seasonally dependent. In the warm season, warmer-than-normal TNA SSTs induce a local increase in atmospheric convection. This leads to a weakening of the North Atlantic subtropical anticyclone and a reduction in precipitation over the United States and northern Mexico, associated with the anomalous southward flow there. In the cold season, a response similar to the warm season over the subtropical Atlantic is identified, but there is also a concomitant suppression of convection over the equatorial Pacific, which leads to a weakening of the Aleutian low and subsidence over western North America, similar to the impact of La Niña although weaker in amplitude. The impact of TNA SST on tropical convection and the extratropical circulation is examined by a set of idealized experiments with a linear general circulation model forced with the tropical heating field derived from the full AGCM.Environmental scienceyk16, rs229, mt2204, nhn2, jam148Lamont-Doherty Earth Observatory, Earth and Environmental SciencesArticlesTropical Oceanic Causes of Interannual to Multidecadal Precipitation Variability in Southeast South America over the Past Centuryhttp://academiccommons.columbia.edu/catalog/ac:134157
Seager, Richard; Naik, Naomi H.; Baethgen, Walter E.; Robertson, Andrew W.; Kushnir, Yochanan; Nakamura, Jennifer A.; Jurburg, Stephanie Denissehttp://hdl.handle.net/10022/AC:P:10552Mon, 20 Jun 2011 00:00:00 +0000Observations, atmosphere models forced by historical SSTs, and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast South America (SESA) since 1901. About 40% of SESA precipitation variability over this period can be accounted for by global SST forcing. Both the tropical Pacific and Atlantic Oceans share the driving of SESA precipitation, with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century. Cold tropical Atlantic SST anomalies are shown to drive wet conditions in SESA. The dynamics that link SESA precipitation to tropical Atlantic SST anomalies are explored. Cold tropical Atlantic SST anomalies force equatorward-flowing upper-tropospheric flow to the southeast of the tropical heating anomaly, and the vorticity advection by this flow is balanced by vortex stretching and ascent, which drives the increased precipitation. The 1930s Pampas Dust Bowl drought occurred, via this mechanism, in response to warm tropical Atlantic SST anomalies. The atmospheric response to cold tropical Pacific SSTs also contributed. The tropical Atlantic SST anomalies linked to SESA precipitation are the tropical components of the Atlantic multidecadal oscillation. There is little evidence that the large trends over past decades are related to anthropogenic radiative forcing, although models project that this will cause a modest wetting of the climate of SESA. As such, and if the Atlantic multidecadal oscillation has shifted toward a warm phase, it should not be assumed that the long-term wetting trend in SESA will continue. Any reversal to a drier climate more typical of earlier decades would have clear consequences for regional agriculture and water resources.Environmental sciencers229, nhn2, web2103, awr2001, yk16, jam148, sdj2110Lamont-Doherty Earth Observatory, International Research Institute for Climate and Society, Ecology, Evolution, and Environmental BiologyArticlesAtmospheric Circulation Response to an Instantaneous Doubling of Carbon Dioxide Part I: Model Experiments and Transient Thermal Response in the Tropospherehttp://academiccommons.columbia.edu/catalog/ac:134171
Wu, Yutian; Seager, Richard; Ting, Mingfang; Naik, Naomi H.; Shaw, Tiffany Annhttp://hdl.handle.net/10022/AC:P:10556Mon, 20 Jun 2011 00:00:00 +0000This study aims to understand the dynamical mechanisms driving the changes in the general circulation of the atmosphere due to increased carbon dioxide (CO2) by looking into the transient step-by-step adjustment of the circulation. The transient atmospheric adjustment is examined using the National Center for Atmospheric Research Community Atmospheric Model Version 3 coupled to a slab ocean model and the CO2 concentration in the atmosphere is uniformly and instantaneously doubled. The thermal structure and circulation response is well established after one year of integration with the magnitudes gradually increasing afterwards towards quasi-equilibrium. Tropical upper tropospheric warming occurs in the first month. The expansion of the warming in the middle and upper troposphere to the subtropics occurs later and is found to be primarily dynamically-driven due to the intensification of transient eddy momentum flux convergence and resulting anomalous descending motion in this region. The poleward displacement of the midlatitude tropospheric jet streams occurs together with the change in eddy momentum flux convergence but only after the intensification of the subpolar westerlies in the stratosphere. The results demonstrate the importance of the tropospheric eddies in setting up the extratropical tropospheric response to global warming.Environmental scienceyw2225, rs229, mt2204, nhn2, tas2163Applied Physics and Applied Mathematics, Lamont-Doherty Earth Observatory, Earth and Environmental SciencesArticlesThe role of linear wave refraction in the transient eddy-mean flow response to tropical Pacific SST anomalieshttp://academiccommons.columbia.edu/catalog/ac:134149
Harnik, Nili; Seager, Richard; Naik, Naomi H.; Cane, Mark A.; Ting, Mingfanghttp://hdl.handle.net/10022/AC:P:10550Mon, 20 Jun 2011 00:00:00 +0000The midlatitude response to tropical Pacific SST anomalies involves changes in transient eddy propagation, but the processes leading to the transient eddy changes are still not clear. In a recent study, we used a series of controlled general circulation model (GCM) experiments in which an imposed tropical Pacific sea-surface temperature (SST) anomaly is turned on abruptly and the response is analyzed in terms of its high- and low-frequency parts, to show that the changes in transient eddies induced by El Niño Southern Oscillation (ENSO) arise from changes in wave refraction on the altered mean flow. In this work, we use a quasi-geostrophic linear model and a linear stationary wave model, to interpret the GCM experiments and obtain the sequence of events that lead from a tropical SST anomaly to the quasi-equilibrium change in the mean and transient atmospheric circulation. The initial direct response of the mean flow is confined to the tropical and subtropical Pacific, similar to what is obtained from a stationary wave model. This tropical–subtropical mean flow change initiates a transient eddy response, which induces a midlatitude mean flow anomaly. The wave–mean flow system evolves towards a state in which the eddy anomalies maintain the mean flow anomalies, allowing them to persist. It is further shown that, while eddy momentum fluxes persistently accelerate and decelerate the subtropical and midlatitude mean flow, the eddy heat flux effect on the zonal mean flow is much more variable, and only marginally significant. The linear quasi-geostrophic model calculations capture the evolution of eddy momentum flux anomalies equatorwards of 60°N quite well, suggesting linear wave refraction can explain the midlatitude ENSO anomalies. However, other processes, like stationary waves or changes in the nonlinear stage of eddy life cycles, are needed to explain the ENSO-related anomalies at high latitudes, polewards of around 60°N.Environmental sciencenh2019, rs229, nhn2, mac6, mt2204Applied Physics and Applied Mathematics, Lamont-Doherty Earth Observatory, Earth and Environmental SciencesArticlesThermodynamic and Dynamic Mechanisms for Large-Scale Changes in the Hydrological Cycle in Response to Global Warminghttp://academiccommons.columbia.edu/catalog/ac:134164
Seager, Richard; Naik, Naomi H.; Vecchi, Gabriel A.http://hdl.handle.net/10022/AC:P:10554Mon, 20 Jun 2011 00:00:00 +0000The mechanisms of changes in the large-scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project phase 3 and used for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report are analyzed by computing differences between 2046 and 2065 and 1961 and 2000. The contributions to changes in precipitation minus evaporation, P − E, caused thermodynamically by changes in specific humidity, dynamically by changes in circulation, and by changes in moisture transports by transient eddies are evaluated. The thermodynamic and dynamic contributions are further separated into advective and divergent components. The nonthermodynamic contributions are then related to changes in the mean and transient circulation. The projected change in P − E involves an intensification of the existing pattern of P − E with wet areas [the intertropical convergence zone (ITCZ) and mid- to high latitudes] getting wetter and arid and semiarid regions of the subtropics getting drier. In addition, the subtropical dry zones expand poleward. The accentuation of the twentieth-century pattern of P − E is in part explained by increases in specific humidity via both advection and divergence terms. Weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to decreased P − E in the ITCZ and to increased P − E in the descending branches of the Walker and Hadley cells. The changing mean circulation also causes decreased P − E on the poleward flanks of the subtropics because the descending branch of the Hadley Cell expands and the midlatitude meridional circulation cell shifts poleward. Subtropical drying and poleward moistening are also contributed to by an increase in poleward moisture transport by transient eddies. The thermodynamic contribution to changing P − E, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity.Environmental science, Climate changers229, nhn2Lamont-Doherty Earth ObservatoryArticles